The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Research to provide more shift-stages of an automatic transmission has been conducted to achieve enhancement of fuel consumption and better drivability, and recently, enhancing fuel efficiency of a vehicle draws more attention due to increase of oil price.
In this sense, research on an engine has been undertaken to achieve weight reduction and to enhance fuel consumption by so-called downsizing, and research on an automatic transmission has been performed as well to provide better drivability and fuel consumption by achieving more shift stages.
In order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may negatively affect installability, production cost, weight and/or power flow efficiency.
As an effort to increase fuel efficiency of an automatic transmission by more shift stages with reduced number of parts, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages has been developed.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements is sometimes attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.
We have discovered that gear ratio spans of recently developed eight-speed automatic transmissions are typically between 6.5 and 7.5, and fuel consumption enhancement is limited.
In addition, we also discovered that when a gear ratio span of an eight-speed automatic transmission rises a level above 9.0, it is difficult to maintain step ratios between adjacent shift stages to be linear, by which driving efficiency of an engine and drivability of a vehicle is undermined.